The present invention relates to a method of manufacturing an electrode for an electromagnetic flowmeter and, more particularly, to a method of manufacturing an electrode to be used in an electromagnetic flowmeter including a ceramic measuring pipe.
A conventional measuring pipe used in an electromagnetic flowmeter is manufactured by stainless steel or the like and its inner surface is coated with an insulating lining. In recent years, however, a measuring pipe formed by sintering a ceramic material such as alumina (Al.sub.2 O.sub.3) as a nonconductive material has been developed. Such a measuring pipe has a higher corrosion resistance than that of the conventional metal measuring pipe with a lining and does not deform at a high temperature. In addition, an electrode or the like can be cast in the measuring pipe of this type. Therefore, this measuring pipe having many advantages has been widely used. A variety of structures have been proposed as an electrode structure for an electromagnetic flowmeter using such a ceramic measuring pipe. One example of the structure is disclosed in Japanese patent laid-open No. 58-501552. In this structure, as shown in FIGS. 1a and 1b, a pair of opposed electrode insertion holes 3 are formed in central portions of a circumferential wall of a cylindrical molded body 4 made of a nonsintered ceramic material (e.g., oxide ceramic). Electrodes 1 are inserted in the electrode insertion holes 3 so that their inner ends face the interior of the molded body 4. In this state, the molded body 4 is sintered at about 1,800.degree. C. to obtain a measuring pipe, and at the same time the electrodes 1 are integrally fixed to the electrode insertion holes 3 by sintering. That is, since a ceramic material contracts (by about 17 to 20% in the case of Al.sub.2 O.sub.3) by sintering, the electrodes 1 and the measuring pipe 4 are integrally formed, and a liquid seal of the electrode insertion holes 3 is obtained. A noble metal such as platinum or a platinum alloy is used as an electrode material. This is because these metals have a satisfactory heat resistance against a sintering temperature of the ceramic material and a thermal expansion coefficient close to that of the ceramic.
Reference numerals 5 denote a pair of excitation coils mounted on the outer surface of the measuring pipe 4.
According to the above method, however, in which the electrodes 1 are inserted in the nonsintered ceramic molded body 4 and integrally fixed to the measuring pipe by sintering, in order to reliably seal the electrode insertion holes 3, a dimensional tolerance between the electrodes 1 and the insertion holes 3 must be strictly set and the circumferential surface of each electrode 1 and the inner surface of each electrode insertion hole 3 must have predetermined surface roughness or higher (i.e., surface undulations must be fine). The electrode insertion holes 3, however, are weak because they are not sintered and therefore are difficult to be subjected to machining for obtaining the necessary surface roughness, resulting in a large number of manufacturing steps and poor yield. In addition, when the electrode 1 is inserted in the nonsintered electrode insertion hole 3, the inner surface of the hole tends to be damaged. Therefore, special care must be taken during assembly. Also, a contraction rate of the ceramic material upon sintering differs in accordance with a lot or a manufacturing time (especially, a season or a weather because of an influence of a temperature or humidity) even if a lot is the same. For this reason, the outer diameter of the electrode 1 and the hole diameter of the electrode insertion hole 3 must be strictly controlled upon each sintering. If the dimensional tolerance is not suitable, fixing of the electrode 1 or the liquid seal becomes defective, or the measuring pipe 4 is cracked or damaged by a stress upon sintering.